It is often desirable to be able to ascertain the intensity of a source of electromagnetic radiation as a function of wavelength, and this is one problem addressed by this invention. Other problems which are dealt with here involve the need to investigate electromagnetic radiation sources of high dynamic range (or intensity range), as well as the need to investigate those having a very broad range of wavelengths. Also, the need to determine the total intensity of any electromagnetic radiation source is addressed here.
Certain instruments and techniques have been used to attempt to solve these problems. These solutions have included using calibrated thermopiles, chemical actinometry, and phototubes. Such prior art solutions to these problems have had various deficiencies, however. The necessary equipment is expensive and has involved using sophisticated and expensive techniques, including various techniques which have been required to calibrate the equipment. Furthermore, the wavelength region and the range of intensities which can be investigated with one set of equipment are often quite limited. Therefore, investigating both high and low intensity electromagnetic radiation sources having wavelengths even in the relatively limited range from vacuum UV to far IR has been inconvenient because various pieces of equipment have had to be employed and various calibrations have had to be done.
The photoacoustic effect has been known since 1880, when it was discovered by Alexander Graham Bell. It consists of illuminating a sample contained in a closed gas-filled cell with chopped light and measuring the resultant acoustic signal. The energy absorbed by the sample is degraded to heat pulses which express themselves in the gas contained in the cell (i.e., the coupling gas) as pressure pulses, (i.e., sound). It has been clearly shown that the intensity of the photoacoustic signal is directly proportional to the power of the radiation incident on the sample. (See Rosencwaig, Opt. Commun., 7, 305 (1973) and Harshbarger and Robin, Accts. Chem. Res., 6, 329 (1973)). These sound signals can be detected by a sensitive microphone and have been used to furnish a variety of types of information about samples (either solids, liquids, or gases) placed in the photoacoustic cell but it has not been recognized that the instrument can be used to analyze the light source.
The present invention employs the photoacoustic effect in an instrument which has outstanding capabilities for analyzing electromagnetic radiation.
It is an object of this invention to measure the intensity of electromagnetic radiation. Another object of this invention is to provide a photoacoustic electromagnetic radiation detecting apparatus.